As semiconductor devices are downsized and highly integrated, the fluctuations of the threshold voltages of the transistors due to statistical fluctuations of the channel impurity becomes conspicuous. The threshold voltage is one of important parameters for deciding the performance of the transistors, and to manufacture semiconductor device of high performance and high reliability, it is important to decrease the fluctuations of the threshold voltage due to the statistical fluctuations of the impurity.
As one technique of decreasing the fluctuations of the threshold voltage due to the statistical fluctuations is proposed the technique that a non-doped epitaxial silicon layer is formed on a highly doped channel impurity layer having a steep impurity concentration distribution.
The following are examples of related: U.S. Pat. No. 6,482,714; U.S. Patent Publication No. 2009/0108350; A. Asenov, “Suppression of Random Dopant-Induced Threshold Voltage Fluctuations in Sub-0.1-μm MOSFET's with Epitaxial and δ-doped Channels”, IEEE Transactions on Electron Devices, vol. 46, No. 8. p. 1718, 1999; Woo-Hyeong Lee, “MOS Device Structure Development for ULSI: Low Power/High Speed Operation”, Microelectron. Reliab., Vol. 37, No. 9, pp. 1309-1314, 1997; and A. Hokazono et al., “Steep Channel Profiles in n/pMOS Controlled by Boron-Doped Si:C Layers for Continual Bulk-CMOS Scaling”, IEDM09-673.
No method for incorporating the proposed techniques described above in the semiconductor device manufacturing processes have been specifically proposed. Especially, new problems which will take place by adopting the above-described techniques in manufacturing processes, and their solving means have not been specifically studied.